AudioMixer: Always do prepareForReformat() in setChannelMasks()

    Always prepareForReformat() when channelmask has changed,
    no matter needing a downmix or not.
    Since ReformatBufferProvider would need to reset when
    channelmask has changed.
    Test: self-tested more then 50 times when a voice call ended
          and there is an incoming Skype/LINE ringtone.
    Bug: 78199280

Test: bug is fixed.
Change-Id: I08c74b00fcbaf8aacef7b1d1c1e840cae347accb
Signed-off-by: Yung Ti Su <andysu@google.com>